The present invention relates to a novel pigment dispersion and an actinic radiation curable type ink jet ink composition, and in more detail to a pigment dispersion in which dispersibility of pigments and dispersion stability are improved, and an actinic radiation curable type ink jet ink using the same.
In recent years, ink jet recording systems have been applied to various printing fields such as photography, various kinds of printings, marking, and special printing such as color filters, since they make it possible to simply and conveniently prepare images at low cost. Specifically, by employing recording apparatuses which eject microscopic ink droplets, and control them, inks which result in improved range of color reproduction, as well as exhibit improved durability and ejection suitability, and special paper sheets of which ink absorbability, color forming properties of colorants and surface glossiness are markedly improved, it has become possible to result in image quality comparable to conventional silver salt photography. Improvements in image quality of the current ink jet recording systems have been achieved only by the integration of ink jet recording apparatuses, inks, and special kinds of paper.
However, in ink jet systems which necessitate special papers, problems surface in which recording media are limited and the cost of the recording media increases. Consequently, many trials have been made in which recording is performed on media to be transferred which are different from special paper sheets, employing the ink jet systems. Listed as specific examples are a phase variation ink jet system which employs a wax ink which is in the form of a solid at room temperature, a solvent based ink jet system which employs ink comprised of rapid drying organic solvents as a major component, and an ultraviolet radiation curable type ink jet system in which crosslinking undergoes employing ultraviolet radiation after printing. However, at present, various problems occur in these systems.
For example, in the phase variation ink jet system, neither sufficient image strength nor fixability is achieved due to use of wax as ink. Further, in the solvent based ink jet system, the following problems occur: unpleasant odors (being an environmental problem) are generated due to high volatility of the ink, and heads are clogged due to dried ink. Further, since ink penetration properties differ depending on the employed recording media, bleeding as well as drying is not stabilized, whereby ink bleeding tends to result.
On the other hand, in recent years, an ultraviolet radiation curable type ink jet system has received favorable attention due to its relatively low unpleasant odor, rapid drying property which makes it possible to carry out recording on non-ink absorptive recording media, compared to the solvent based ink jet systems. For example, Japanese Patent Publication No. 5-54667, Japanese Patent Publication Open to Public Inspection No. 6-200204, and Japanese Patent Publication Open to Public Inspection (under PCT Application) No. 2000-504778 disclose ultraviolet radiation curable type ink jet inks. In any of these methods, employed are acryl monomers as a polymerizable compound. However, in view of a decrease in viscosity to achieve stable ink ejection, an increase in photo-speed, and closer adhesion to recording media, selected as polymerizable monomers which are employed in ink are those which irritate skin or induce allergic reactions. Consequently, they are highly toxic to customers and operators. Further, Japanese Patent Publication Open to Public Inspection (under PCT Application) No. 2000-504778 discloses a method in which either alkoxyacrylates or polyalkoxyacrylates, which exhibit relatively high safety standards, are incorporated in an amount of 80-95 percent by weight. However, a structure, in which either monofunctional or multifunctional monomers are incorporated in an amount of a minimum of 80 percent by weight, results in problems in which sufficient adhesion to recording media is not achieved, ink bleeding occurs, and unpleasant odor due to unreacted monomers is generated with respect to a wide range of recording media, such as highly permeable recording media or highly wettable and easily bleeding polyethylene terephthalate films.
Assumed as factors in which sufficient photocuring speed is not achieved are bleeding of printed images before the ink is cured and penetration of the ink into porous recording medium. In addition, the aforesaid penetration has enlarged the problem of the residual unpleasant odor due to the unreacted monomers. Considered as methods which aid in an increase in photocuring speed are means in which the intensity of a radiation source is increased and exposure time is increased. However, all these methods have resulted in an inevitable increase in cost of the integral apparatuses.
As noted above, the radiation curable type ink jet system, which employs ultraviolet radiation as one means of radiation, exhibits excellent features compared to other ink jet systems. However, in view of safety and apparatus cost, the present radiation curable type ink jet system is inferior to the water-based ink jet system. Consequently, urgently demanded is development of problem-solving methods.
Listed as means to overcome the above problems is an ultraviolet radiation curable type ink jet system employing compounds which are cured via cationic polymerization. For example, Japanese Patent Publication Open to Public Inspection No. 8-143806 discloses an actinic radiation curable type ink composition employing an oxetane ring-containing compound (hereinafter referred to as an oxetane compound). Oxetane compounds neither irritate skin nor induce allergic reactions. Further, the oxetane compounds exhibit features in that polymerization is not inhibited due to the presence of oxygen in the same manner as radical polymerization of acryl monomers, and it is also possible to cure the ink employing a radiation source at a relatively low output. However, problems still exist in which pigment dispersibility and dispersion stability are inferior.